Mobile communication by means of cellular networks is an integral part of modern life. One example of cellular networks is the Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) technology.
The LTE technology is a scheduled technology where an access node—referred to as evolved node B (eNB) in the LTE framework—allocates time/frequency resources (resource blocks) for uplink (UL) and downlink (DL) communication. The LTE technology employs Transmission Time Intervals (TTI) offering a resource granularity of 1 millisecond; the TTIs are implemented by subframes.
Where a terminal requires to transmit UL payload data, it sends a UL transmission request and receives a corresponding UL transmission grant. Likewise, where the eNB requires to transmit DL data, it sends a DL assignment to the terminal to announce the DL data. Such techniques are referred to as scheduling.
In order to protect communication of data on the radio link, the LTE technology implements a Hybrid Automatic Repeat Request protocol (HARQ). Firstly, HARQ employs Forward Error Correction (FEC) by encoding data communicated in messages. By adding a respective redundancy bits according to a coding scheme, erroneously received data packets can be detected. And by adding redundancy bits to data packets according to a coding scheme, bit errors occurring during transmission can be healed to some extent. Secondly, HARQ handles erroneously received data on a radio access level and is typically implemented by a Medium Access (MAC) layer of a transmission protocol stack of the terminal and the eNB, respectively. In detail, according to the LTE technology, a payload data message communicated on the radio link in subframe n is positively or negatively acknowledged in subframe n+4. Where the payload data message is negatively acknowledged (negative acknowledgment; NACK), retransmission of the payload data message—which in some examples may now be encoded according to a different redundancy version—is implemented in subframe n−8. Such retransmission facilitates successful reception of the payload data message. Details of the HARQ protocol in the LTE technology are illustrated in the 3GPP Technical Specification (TS) 36.321 V.12.7.0 (2015-09-25).
Implementing the HARQ protocol employing different redundancy versions for different retransmission attempts enables a certain degree of time diversity and, thus, increases the likelihood of successful transmission. Thereby, the total coverage of the cellular network may be increased.
However, it is sometimes desired to even further increase the coverage. A set of features where a comparably large coverage is achieved is referred to as Coverage Enhancement (CE). CE technology is envisioned to be applied for Machine Type Communication (MTC) and the Narrowband Internet of Things (NB-IoT), sometimes also referred to as NB-LTE. These techniques may be based on the LTE technology to some extent and may reuse some of the LTE concepts.
The MTC and NB-IoT techniques are envisioned for so-called massive deployment scenarios where a large number of terminals is connected to cellular networks. E.g., it is expected that the number of terminals per access node may increase by a factor of 10-100 over the next years. In such a scenario, it is expected that up to 300,000 devices are required to be served per access node.
A key feature of the CE technology is to repeat each redundancy version of encoded data within the HARQ protocol a number of times (transmission repetitions). Such a repetition may be “blind”, i.e., not in response to a respective retransmission request, but rather preemptive. Here, it is typically assumed that the repetitions of messages carrying one and the same redundancy version are implemented by a bundled transmission set of messages communicated in consecutive/subsequent subframes of a channel implemented on the radio link, see, e.g., 3GPP Technical Report (TR) 45.820 V13.0.0 (2015-08), Section 6.2.1.3. By employing a bundled transmission set, a likelihood of successful transmission can be increased even in scenarios of poor conditions of transmitting and/or receiving (communicating) on the radio link. Thereby, the coverage of the cellular network can be significantly enhanced—even for low transmission powers as envisioned within the MTC and NB-IoT domain. This facilitates the CE technology.
Typically, the number of messages including data encoded according to a given redundancy version is preconfigured by a bundling policy. The bundling policy may be chosen according to certain properties of the radio link and/or the terminal. The bundling policy may be (semi-)persistently employed for a certain time duration.
However, such techniques face certain restrictions and drawbacks. Due to the large number of transmission repetitions typically required, the traffic load on the radio link can be high.